1. Field of the Invention
The present invention relates to methods of preparing metal nanocrystals.
2. Description of the Related Art
Metal nanocrystals, which are also known as quantum dots, have radii of several nanometers, which are smaller than the Bohr exciton radii. Quantum dots have lots of electrons, but the number of free electrons in the quantum dots is limited to the range of 1 to 100.
Since the electrons of the quantum dots have discrete energy levels, the quantum dots have intrinsic electric and optical properties which are different from bulk metals, which have continuous bands of energy levels.
For example, in a conventional semiconductor process, various conductors and insulators are combined to obtain a semiconductor having a certain band gap. However, the energy level of a semiconductor quantum dot varies according to the size of the quantum dot, and thus the band gap can be controlled by simply changing the size of the quantum dot. In addition, a so-called “Coulomb Blockade” can occur. A “Coulomb Blockade” occurs when an electron in a quantum dot blocks a new electron trying to enter the quantum dot. These “Coulomb Blockades” are triggered when the energy required to add electrons is not uniform and changes in stages in the quantum dot. This is different from adding an electron in a bulk phase semiconductor.
Thus, metal nanocrystals having these properties can be used in magneto-optic, thermoelectric, and magneto-electric operations. More particularly, metal nanocrystals can be used in various fields such as information storage media, single electron transistors, light emitting devices (LEDs), biomolecule labeling, and solar batteries.
Metal nanocrystals can be prepared using vapor phase methods, liquid phase methods, etc. Examples of vapor phase methods include metal-organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE). When metal nanocrystals are prepared using the vapor phase method, high degree of crystallinity can be achieved, but the density and uniformity of the metal nanocrystals cannot be easily regulated.
On the other hand, the liquid phase method includes growing quantum dot crystal precursor materials into quantum dot crystals in an organic solvent using a wet chemical method. Using this method, the properties of the nanocrystals can be easily controlled by adjusting reaction conditions and the nanocrystals can be easily prepared. However, when metal nanocrystals are prepared using the liquid phase method, yield is low.